A recirculating processing oven heater disclosed by U.S. Pat. No. 4,098,567, which is assigned to the assignee of the present invention, incorporates a catalytic converter downstream from a direct fired burner so as to remove unburned combustible gas components from the processing gas while providing additional heating thereof prior to delivery to an associated oven. This heater includes an inner housing defining a first gas flowing path along which the burner and catalytic converter are disposed and an outer housing that cooperates with the inner housing to define a second gas flow path along which bypass processing gas flows without any direct heating thereof by the burner. A mixing chamber defined by the outer housing portion is located downstream from a conical mixing member that points upstream toward the catalytic converter at the downstream end of the first gas flow path so as to deflect gas outwardly and thereby provide mixing of the gas flowing along the two paths. Such a heater construction allows the gas passing through the catalytic converter to be in the range of about 600.degree. to 1200.degree. F. so that effective catalytic operation takes place while still delivering heated processing gas after mixing in a lower more usable range of about 150.degree. to 500.degree. F.
U.S. Pat. No. 3,604,824 discloses a thermal incineration unit which incorporates dual flow paths as well as a heat exchanger downstream from the heater burner so as to remove heat from the gas after the gas has been at a sufficiently high temperature for a sufficient period of time for thermal incineration to remove combustible pollutants from the gas.
U.S. Pat. No. 2,985,438 discloses combustion apparatus including a first gas flow path along which the burner thereof is disposed and second and third bypass gas flow paths concentric with and located outwardly of the first path. At the downstream end of the three flow paths, adjustable louvers deflect heated gas from the first path outwardly toward bypassed gas that flows along the second and third paths in order to provide mixing of the gas within a downstream mixing chamber.
U.S. Pat. No. 3,146,821 discloses a furnace heater including burner and bypass gas flow paths as well as a heat recuperator which removes heat from return gases and supplies this heat to the gas that flows through both the burner and bypass paths.